pgsql: Parse pg_hba.conf in postmaster, instead of once in each backend

Log Message:
-----------
Parse pg_hba.conf in postmaster, instead of once in each backend for
each connection. This makes it possible to catch errors in the pg_hba
file when it's being reloaded, instead of silently reloading a broken
file and failing only when a user tries to connect.

This patch also makes the "sameuser" argument to ident authentication
optional.

Modified Files:
--------------
pgsql/doc/src/sgml:
client-auth.sgml (r1.106 -> r1.107)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/client-auth.sgml?r1=1.106&r2=1.107)
pgsql/src/backend/libpq:
auth.c (r1.167 -> r1.168)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/auth.c?r1=1.167&r2=1.168)
crypt.c (r1.74 -> r1.75)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/crypt.c?r1=1.74&r2=1.75)
hba.c (r1.166 -> r1.167)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/hba.c?r1=1.166&r2=1.167)
pg_ident.conf.sample (r1.7 -> r1.8)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/pg_ident.conf.sample?r1=1.7&r2=1.8)
pgsql/src/backend/postmaster:
postmaster.c (r1.562 -> r1.563)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c?r1=1.562&r2=1.563)
pgsql/src/include/libpq:
hba.h (r1.48 -> r1.49)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/hba.h?r1=1.48&r2=1.49)
libpq-be.h (r1.66 -> r1.67)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/libpq-be.h?r1=1.66&r2=1.67)